Surface hydroxylation of TiO2 yields notable visible-light photocatalytic activity to decompose rhodamine B in aqueous solution
Introduction
Under light irradiation, a semiconductor photocatalyst produces oxidative radicals, such as , , and , which attack organic pollutants in water and air for decomposition [1]. While TiO2 has been the most widely investigated semiconductor photocatalyst [1], [2], [3], [4], [5], [6], [7], [8], [9], it requires UV light since the band gap-excited electron-hole pairs across the band gap (E>3.2 eV; λ<388 nm) are needed to form the oxidative radicals. In order to exploit the abundant Vis-light part (about 40%) of sunlight, the process of doping the TiO2 with various cations or anions [2], [3], [4], [5], [6], [7], [8] has been extensively researched. This approach employs band gap narrowing or the creation of defect energy levels [7] to form the charge pairs by absorbing Vis light (wavelengths of approximately 400–500 nm).
The photosensitization is the process of initiating a reaction through the use of a light-absorbing substance (e.g., dye) and transferring the energy to the desired reactants. In the present study, we focus on the fact that TiO2 itself shows a weak photocatalytic activity under Vis-light irradiation [9], [10], [11], [12] through the photosensitization. Unlike the UV mechanism, the Vis-light mechanism (photosensitization) does not involve the electron excitation from the valence band to the conduction band (CB). Instead, it starts with electron injection from the Vis-light-activated organic compound (rhodamine B in the current work) to the CB of semiconductor [9], [10], [11], [12]. The oxidized (electron-lost) rhodamine B (RhB+) is subsequently decomposed via multistep photooxidation processes [11]. The electron which was injected into the CB of TiO2 then escapes to reduce the dissolved oxygen, which forms an radical. This radical then generates other informative oxidants such as and by a sequence of reactions, both of which attack rhodamine B (RhB) and/or RhB+ together with for decomposition [11].
The ball mill process has been used for the mechanochemical synthesis of TiO2 [13]. Also, the phase transformation of the TiO2 during the high-energy ball mill process has been the topic of research [14]. In the present study, we hypothesized that the reforming of the surface state of TiO2 by providing appropriate milling energy may alter the electron injection and/or escape characteristics to/from the CB of TiO2, thereby facilitating the Vis-light mechanism. In order to check this hypothesis, an experiment shown in the experimental section was performed.
Section snippets
Sample preparation and characterization
10 g of as-received TiO2 powder (Degussa, P-25) was shifted to a polyethylene bottle (56 mm in diameter and 250 ml in nominal volume) with 70 ml of distilled water and approximately 500 g of zirconia balls (approximately 5 mm in diameter). A relatively mild mechanical energy was provided to the TiO2 powder by ball milling at 56 rpm for varying time. The balls traveled along the height direction of the bottle. After ball milling, distilled water was added to the separated slurry and stirred for 30 min
Photocatalytic activity
Fig. 1(a) shows the temporal change of the absorbance spectrum from the aqueous solution with TiO2 powder ball milled for 72 h. (The spectra of the solution were obtained after centrifugal separation of the ball-milled TiO2 powder). The absorbance peak centered at 555 nm results from the RhB molecules in the solution. As the irradiation time increases, the peak area decreases, which accompanies with a hypsochromic shift of the peak. The hysochromic shift results from the formation of the
Conclusion
As this study has shown, simple ball milling of TiO2 forms a OH/H2O-rich surface of TiO2. Based on the well-documented existing works, this layer of surface hydroxylation has been interpreted to facilitate the charge transfer across the surfaces of TiO2 in the form of hydroxyl radical ({>TiOH•}+), increase the path of the back electron transfer, enhance the surface adsorption of RhB molecules, and promote the surface adsorption of dissolved oxygen molecules. The combination of these sources
Acknowledgment
This work was supported financially by the Basic Science Research Program (2010-0004150), funded by the Ministry of Education, Science and Technology through the National Research Foundation of Korea (NRFK). It was also co-supported by the Geo Advanced Innovative Action (GAIA) Project (No. RE201202040), funded by the Ministry of Environment of Korea through the Soil Environment Center at Korea Environmental Industry & Technology Institute (KEITI). The portion of professor Jung was also
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